1. Field of the Invention
The invention relates to electronic communication and, more particularly, to the synchronization of a pseudo-random noise sequence in a direct-sequence spread spectrum receiver.
2. Description of the Related Art
Direct-sequence spread-spectrum (DSSS) radio communication is accomplished with a carrier wave that is modulated by an information signal into a narrowband signal. The narrowband signal is further modulated by a spreading code, which broadens its spectrum, into the transmitted spread-spectrum signal. The spreading code is a pseudo-random noise (PN) sequence of symbols, called chips, that each have a short duration in comparison with the symbols of the information signal. Although the modulation with the PN sequence is typically performed after the carrier has been modulated to bear the information signal, it can alternatively be performed on the information signal before modulation onto the carrier.
In some communication systems that employ PN sequences with durations equal to or shorter than the symbol duration, the information signal is recovered by filtering the PN frequency components from the received spread-spectrum signal. In a typical receiver, however, the received spread-spectrum signal is despread by demodulating the PN sequence from it. Thus, these receivers need to generate a local copy of the PN sequence. The receiver demodulates the spread-spectrum signal with the locally generated PN sequence to extract the narrowband signal; it then demodulates the narrowband signal to reconstruct the information signal.
The despreading requires the receiver's locally generated PN sequence to be synchronized with the PN sequence modulated into the received signal. If the mismatch in timing is large (more than 1 chip), the despreading can not effectively restore the narrowband signal. The term "timing recovery" refers to the process of ensuring that the receiver's locally generated PN sequence is substantially synchronized with the PN sequence modulated into the received signal. There are two stages in the timing recovery of the PN sequence. In the first, the acquisition stage, the receiver acquires a coarse synchronization, generally to within 1/2 chip, between the receiver's locally generated PN sequence and the PN sequence of the received signal. Parallel-search (maximum likelihood), serial-search, or sequential-estimation circuits are typically used for coarse PN timing recovery. Delay-locked loops or tau-dither loops are then employed in the tracking stage, which maintains a fine PN synchronization. The tracking stage recovers a PN timing that is adequate for demodulating the narrowband signal from the received spread-spectrum signal.
The receiver can be improved by supplementing the tracking circuits with an independent system for recognizing a loss of PN synchronization. Upon detecting a degraded synchronization, the receiver can revert to the acquisition stage of the synchronization before an excessive amount of information is lost from improper demodulation. A procedure for recognizing degraded PN synchronization is thus a valuable addition to a spread-spectrum receiver.